During the previous year, we have perfected a new in vitro labeling procedure to produce 125I-RNA with very high specific activity (20-200 x 10 to the 6th poer dpm/micrograms g) and perfectly normal structure and function. During the next year's research, we propose to use 125I-mRNA from chick embryos to ask two questions: (1) is the protein factor, mesodermal inductor, which has the capabilities of inducing the development of mesodermal structures in competent tissues, capable of initiating protein synthesis, and (2) to protein synthesis initiation factors from embryonic and adult chicken tissues have the capabilities to discriminate between different mRNA preparations? To answer these questions, we will build an appropriate mRNA binding assay. The binding assay will allow us to study the role that the regulation of protein synthesis plays during early vertebrate development.